Sea otters, sea urchins and algae

When Europeans settled the Alaskan coast in the 19th century, they created a trade in sea otter pelts that led to the disappearance of the species in the region. In the absence of the otters, the sea urchins they love swarmed. They have eaten up all the kelp, the large brown seaweed that can grow to tens of metres in length and line the coasts of the North Pacific. The kelp forests have disappeared, leaving small fish unprotected and the coastline bare to the ocean waves. This “ecological cascade” is an emblematic example of the massive impact of the extermination of a single species on an entire seascape.

Fortunately, the transformation is reversible: as soon as otters were reintroduced to Alaska in the late 1960s, they went back to fishing for sea urchins. Where they thrive, the kelp forests are less ferociously browsed, and gradually recover. All’s well that ends well? Not completely, because, apart from sea urchins, otters love to eat shells. They bring them to the surface and, floating comfortably on their backs, they smash them with pebbles. From the shore, otters can easily be seen treating themselves to a platter of seafood, then grooming themselves vigorously with their offspring. Even the most hardened of scientists describe the sight as “wonderfully adorable”. The First Peoples of southern Alaska, whose livelihoods include shellfish harvesting, are not entirely enchanted. Local communities see the otters as competitors, and have obtained hunting licences, greatly reducing the populations of the small marine mammals near the village of Sitka in the Alexander Archipelago.

Is it impossible for fishermen and otters to live together? Researchers from the University of California at Santa Cruz have been looking into this question (1) . For three decades, they have studied otters, sea urchins and kelp near Sitka and in Torch Bay, an uninhabited area in southern Alaska. They have traced the ups and downs of the ecological cascade associated with otters: with them come kelp forests. However, beyond these major trends, they identified a mosaic of coastal micro-habitats, within which otters have very restricted territories: females spend their entire lives within a 10-25 km radius, and the animals avoid areas where they are likely to encounter their predators, killer whales and great white sharks.

In the infinite landscapes of Alaska, an ecological balance is therefore likely to be established naturally; local communities do not necessarily have to choose between otters and shellfish, ecotourism and fisheries. As the authors of the study note, this hypothesis is reinforced by “archaeological evidence that indigenous people in the Pacific Northwest had access to distinct areas where shellfish and sea otters were abundant”.
(1) Gorra et al. (2022) Southeast Alaskan kelp forests: inferences of process from large-scale patterns of variation in space and time. Proc. R. Soc. B 289: 20211697.

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